Does NASA use time? How NASA keeps time in space

Yes. NASA runs on precise time. Mission control and the Deep Space Network use UTC, the atomic-clock world standard; the International Space Station also runs on UTC (GMT). Because radio signals travel at the speed of light, NASA turns timing into distance: measuring how long a signal takes to reach a spacecraft and come back tells navigators where it is.

The clocks NASA uses

When it happened vs. when we found out

NASA's own example from the Cassini mission: Cassini began sending data from its first close flyby of Saturn's moon Titan at 00:16 UTC spacecraft time, and the signal reached Earth at 01:30 UTC, 6:30 p.m. Pacific time in mission control. Everything the team saw had already happened more than an hour earlier. From Saturn, NASA says the one-way light time ranges from about 1 hour 14 minutes to 1 hour 24 minutes.

Voyager 1: one light-day from Earth

NASA's Voyager 1, launched in 1977, is the most distant human-made object. According to NASA, it reaches one light-day from Earth on November 18, 2026, at 06:16:07 UTC (12:16 a.m. Central time). From then on, a command sent from Earth takes a full day to arrive, and the answer takes another day to come back.

Time turns into distance

Light travels about 300,000 km per second, so a clock error becomes a distance error: 1 millisecond ≈ 300 km, 1 microsecond ≈ 300 m, 1 nanosecond ≈ 30 cm. NASA's Deep Space Network sends a signal, the spacecraft returns it, and the round-trip time gives the distance. That is why the network uses hydrogen-maser atomic clocks on the ground.

The Deep Space Atomic Clock

To let spacecraft navigate without waiting for Earth, NASA's Jet Propulsion Laboratory built the Deep Space Atomic Clock (DSAC), a toaster-sized mercury-ion clock. It launched on a SpaceX Falcon Heavy in June 2019, ran in orbit for about two years and was switched off in September 2021 after proving it was stable enough for one-way navigation. A successor, DSAC-2, is planned to fly on the VERITAS mission to Venus. See how atomic clocks work.

For a real example, see the full timeline of Artemis II, which flew around the Moon in April 2026 in 9 days, 1 hour and 32 minutes.

Time on the Moon: Coordinated Lunar Time

Clocks run at slightly different rates in different gravity and at different speeds (Einstein's relativity). An atomic clock on the Moon's surface would gain about 56 microseconds per day compared with one on Earth. NASA's Cheryl Gramling notes that at the speed of light that is the length of about 168 football fields, more than enough to confuse navigation. A White House directive of April 2, 2024 asked NASA to deliver a plan for Coordinated Lunar Time (LTC) by December 31, 2026, working with international partners. LTC will be built from atomic clocks on or near the Moon and stay traceable to UTC. See relativity and time dilation.

Time on Mars

A Mars day, called a sol, lasts 24 hours 39 minutes 35 seconds. Rover teams have lived on "Mars time" during early mission operations, shifting their workday about 40 minutes later every Earth day so they work while the rover is awake. A one-way message to Mars takes roughly 3 to 22 minutes, so rovers drive themselves between commands.

GPS: the same idea on Earth

GPS satellites carry atomic clocks and broadcast the exact time each signal left. Your phone measures when signals arrive from at least four satellites and works out both its position and its own clock error. GPS time started on January 6, 1980 and has no leap seconds, so it is now 18 seconds ahead of UTC (live GPS time; how GPS uses time). Air travel depends on it heavily; see how time runs air travel.

Sources

• NASA: What time is it in space?
• NASA: lunar time standard
• White House OSTP: Celestial Time Standardization Policy (April 2, 2024)
• EarthSky: Voyager 1 reaches 1 light-day (citing NASA)
• BBC Science Focus: ISS time zone
• Wikipedia: Deep Space Atomic Clock

Last updated October 22, 2026. Found an error? Tell us.